Next Wednesday, 6 June, isn't just the annual anniversary of D-Day. It's the last chance this century to see the Transit of Venus, one of the rarest and most important of astronomy's major - and generally understandable - events.

The planet Venus will be visible as a tiny black dot crossing the face of the sun, a process which allows the experts to fix the place of the planet Earth amongst the galaxies and within our solar system. This happens twice within approximately eight years every 120 years or so, and this is the second of a pair. The first of the two was on June 8 2004.

And it also shines glory on Salford. Look what playwright Eric Northey came across when he was walking through Lower Broughton in Salford. This plaque.

It commemorates a 17th century haberdasher from the area, William Crabtree, who was also an amateur astronomer. He and a friend, Jeremiah Horrocks, were the first two people to see the Transit of Venus, in 1639, after they had calculated when it would occur. As Northey says:

It was a colossal achievement for Salford over 400 years ago, in what was a hamlet in an obscure part of north-west Lancashire. It got me wondering: how did Crabtree get hold of Keplers' Tables, which were published in 1632, so he could do the mathematics to observe the transit?

And, incidentally, correct Kepler, who also predicted the phenomenon but got the year wrong.

Northey was so intrigued by the plaque that he decided to research Crabtree's life and that of his collaborator, Jeremiah Horrocks, both of them ranked among the 'giants' on whose shoulders Sir Isaac Newton modestly said that he stood to make his own world-changing discoveries. From that research, he has written a play called The Transit of Venus which will be premiered in the 24/7 fringe theatre festival in July in Manchester.

The last transit, in 2004. The next one isn't due until 2117 although it won't be visible from Europe. That doesn't happen again until 2125. Photograph: Murdo MacLeod for the Observer Murdo MacLeod/Observer

Crabtree's trade allowed him to travel across Europe which is probably how he obtained a copy of Kepler's Tables. Interestingly, north west England at the time was also a hotbed of scientific discovery. These need revisiting. As Northey says:

You wouldn't know from reading the history of science in Britain that this area was a central enclave of astronomical discovery. Four great astronomers were living locally. Townley in Burnley, Gascoyne in Leeds, Horrocks in Preston and Crabtree in Salford.

Crabtree and Horrocks were educated men; both could read and speak Latin which was important for communicating with other scientists across Europe. But the chaos of the English civil war in the following decade meant that much information about them was lost and their discovery of the Transit of Venus was not published until the 1680s.

Northey decided to write a historical drama about Crabtree because so little is known about him:

I did not want to write a propaganda piece but I wanted to comment on some contemporary themes.

Throughout the play, there is a discourse about rational thought amid the rising level of violence as 17th century England geared itself up for a bloody conflict.. Northey, who has worked in Nigeria, and saw similar religious violence going on there, says that the theme remains relevant:

It's a key problem even today: why do people who believe in one religion want to kill those who have a different faith? Crabtree's discovery would also have been seen as heresy. Standing up against received wisdom at that time could have cost him and Horrocks their lives.

In the play Crabtree is pushed towards agnosticism as he witnesses the hanging of an elderly woman as a witch.

Alyx Tole, the director, has worked to develop the role of Crabtree's daughter, Jenny, who is taught mathematics and Latin in the play and becomes involved with her father and Horrocks in their scientific work. Alyx defends this imaginative addition:

Women of that generation were educated and as many men died during the Civil War, those women including daughters and wives were left to carry on the family business.

Northey is aware that he has taken liberties with the characters of Crabtree: He was a genius but he died at 35 and so little was known about him, and few historical records have survived that era. He made a discovery that could have changed the history of this country but because of the English Civil War it was hidden for another 50 years.

The Transit of Venus will premiere at the Buxton Fringe Festival on 16-18 July and the 24/7 Theatre Festival at New Century Hall, Manchester from 16-18 July.

Information on observing the transit - do not look directly at the sun - can be found here.